OPS mélange: a new term for mélanges of convergent margins of the world

Ocean plate stratigraphy (OPS) is essential to understanding accretionary wedges and complexes along convergent plate margins. Mélanges within accretionary wedges and complexes are the products of fragmentation and mixing processes during and following OPS accretion. A new term, ‘OPS mélange’, is proposed here for mélanges composed mostly of blocks of OPS with an argillaceous matrix, and for a mixture of mélanges of multiple origins with either broken or coherent formations. An OPS mélange results from the fragmentation and disruption of OPS, without admixing of other components. Three major types of OPS mélange can be distinguished on the basis of their components: turbidite type, chert–turbidite type, and limestone–basalt type. These three types potentially form similar mélanges, but they are derived from different parts of the OPS, depending on the level of the decollement surface. The concept of ‘OPS mélange’ can be applied to most of the mélanges in accretionary prisms and complexes worldwide. In addition, this proposal recognizes a distinction between processes of fragmentation and mixing of OPS components, and mixing of ophiolite components, the latter of which results in serpentinite mélanges, not OPS mélanges. Mélanges composed of OPS sequences occur worldwide. The recognition of OPS mélanges is a key aspect of understanding tectonic processes at convergent margins, which result in mélange formation in orogenic belts globally.     

L'énergie nucléaire : obstacles et promesses

Nuclear energy has distinctive merits (sustainable resources, low costs, no greenhouse gases) but its development must overcome serious hurdles (fear of accidents, radiophobia, waste management). The large unit size of present-day reactors is compatible only with large electrical grids, and involves a high capital cost. Taking into account these different factors, the paper outlines how nuclear energy may contribute to the reduction of greenhouse gases, and which are the most promising developments. To cite this article: P. Bacher, C. R. Geoscience 335 (2003).     

Géologie des gisements de fer: Métallogénie ou Géochimie?

Résumé L'une des voies actuelles de la recherche menée pour comprendre les caractéristiques des gisements minéraux est de les considérer comme des anomalies de l'écorce terrestre.Une approache nettement plus fertile peut être entrevue dans un contexte géochimique. Un gisement de fer apparait comme l'impact de phénomènes géologiques élémentaires favorables situés à diverses échelles, sur les propriétés spécifiques du fer. Les gisements n'apparaissent que parce qu'il existe une hétérogénéité de la distribution des éléments chimiques dans l'écorce terrestre. L'analyse de cette hétérogénéité doit être regardée comme l'analyse des différentes formes de dilution. Cependant, chaque niveau de dilution est sous la dépendance de causes particulières et la poursuite de la concentration vers des échelles géométriques de plus en plus grandes est déterminée par ces causes favorables de plus en plus nombreuses dont la probabilité de rencontre est de plus en plus faible.

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Many geologists want today to explain the characteristics of mineral deposits as anomalies in the earths crust. A more fruitful approach may be the consideration of the geochemical context. An iron deposit appears thus as the combination of favourable elementary geological phenomena with the specific properties of iron. There are deposits which may be attributed only to the heterogeneity in the distribution of elements through the earths crust. The analysis of this heterogeneity is an analysis of the different features of dilution. However, for each dilution level there are particular causes, and the continuation of a certain concentration towards larger geometric scales is determined by a growingly larger number of favourable causes and thus by causes whose probability of coincidence is increasingly smaller.

     

Les minéraux ferrotitanés du littoral sénégalais: Caractérisation et recherche de leur origine

Iron-titanium rich grains (‘ilmenites’ to a various degree of weathering) occuring along the coast line of Senegal and deriving from various parent units are characterized from their size, texture, magnetic and geochemical properties. Both the corresponding data and their multivariate analysis lead to the conclusion that the ‘ilmenite’ grains derive from various sources, mainly local ones, except in the area of Saint-Louis where occur more distal grains from Mauritania. In the North of Cap Vert and in the South of Gambia, the ‘Continental Terminal’ formation represents the main source of the ‘ilmenite’ grains. Along the ‘Petite Côte’, ‘ilmenites’ derive mainly from the Maastrichtian formations of the ‘Massif de Ndiass’. Volcanic formation of the Cap Vert area does not represent a major source, except in the area of the extremity of the peninsula. The old ferricretes do not significantly contribute to the recent accumulation. The ‘ilmenites’ concentration along the beach lineis controlled by the different sedimentary cycles; the nouakchottian transgression beeing probably the most important event. In the Senegalese basin, a significant amount of the ‘ilmenites’ originated from the ‘Continental Terminal’ formation appears to derive from basic metamorphic rocks of the Mauritanide orogen. In the northern part of the basin, ‘ilmenite’ assemblage contains grains suggesting intermediate to acidic magmatic parental rocks. The calc-alkaline igneous complex from the structural units emplaced in the late Paleozoic on the NW margin of the Mauritanide orogen, and the Reguibat uplift, are two possible sources. The ‘ilmenite’ association found in the Maastrichtian formation probably derives from various paleozoic formations but its primary origin can not be deciphered.     

Environmental conservation and the production of new territories: the example of French départements

Very important initiatives have been taken and policies have been adopted in the European Union to protect areas of great importance for threatened species and habitats. Protected areas differ broadly in terms of category, natural conditions and administrative organisation, from international initiative such as Biosphere Reserves, European ones with Natura 2000 network until the institution of national and regional protected areas. In France, the administrative subdivisions known as “départements” were created with the French Revolution in the end of the eighteenth century; in 1985 an original and autonomous procedure to establish special protected areas, called “Sensitive Natural Spaces” (SNS) was devolved to them. The scope of this paper is to present an overview of these devolved powers which enable French départements to create protected areas and to levy a departmental tax on sensitive natural spaces (DTSNS). We statistically studied some parameters by multivariate methods in order to explain the choices of this policy by the départements. The huge variations in the way these powers are implemented prove the development of new environmental territories.     

Geochemistry of gas emanations: A case study of the Réunion Hot Spot,Indian Ocean

The results of analysis of natural emanations in Réunion Island show a clear magmatic origin for CO₂ and He, while N₂ and Ar are predominantly derived from the atmosphere. The distribution of magmatic gases in the Piton des Neiges massif fits the local volcanotectonic context well and suggests that the areas concerned are still subject to volcanic activity at depth. A simple method is proposed for correcting gas concentration and isotope composition for water degassing. In doing so, the isotope and elemental (C, He) composition of gases is homogeneous for the two volcanoes. The isotope ratio of He (12.5 ± 0.5R/Ra) in the present discharges is in agreement with the results of previous studies on rocks of various ages from the two volcanoes. The isotope ratio of C(δ¹³C= −5 ‰ to −4 ‰ vs PDB) and the C/³ He ratio (∼4 × 10⁹) are similar to those found in other Hot Spot volcanic systems such as Kilauea (Hawaii) and Hengill (Iceland). These similarities suggest comparable volatile history for the respective mantle sources, the main differences being in the relative proportions of radiogenic ⁴He. In detail, Hot Spots appear enriched in C having a light composition with respect to MORB, possibly due to the addition of a C-rich (e.g. subducted) component, in addition to a relatively undegassed, ³He-rich, component.     

Double evaporitic décollements: Influence of pinch-out overlapping in experimental thrust wedges

This article focuses on how deformation and displacements are transferred between two décollements located at different stratigraphic levels by means of analogue modeling using brittle/viscous, sand/silicone systems. We present results from ten analogue models, in which we varied key parameters, such as the amount of horizontal offset or overlap between the pinch-outs of the upper and lower décollements, the total amount of shortening, and the planform geometry of the upper décollement. Results indicate that (i) structures root onto the basal and upper décollement, defining an inner and an outer domain and (ii) the offset/overlap of the décollements controls the geometry of the transition zone located between the two décollements, the propagation of deformation into the foreland both in space and time, and the deformation style and kinematics in the different domains of the model. When the pinch-out of the upper décollement is at an angle with the backstop, oblique structures form, and the geometry and propagation-mode of the structures change progressively along-strike. We compare our experimental results with other silicone/sand analogue models and with the natural examples of the Southern Pyrenees, where Upper Triassic and Eocene–Oligocene syn-tectonic evaporites acted as basal and upper décollements, respectively.     

Thermal transformations of akaganéite and lepidocrocite to hematite: assessment of possible precursors to Martian crystalline hematite

We examine the possibility that crystalline hematite (α-Fe₂O₃) deposits on Mars were derived from the precursor iron oxyhydroxide minerals akaganéite (β-FeOOH) or lepidocrocite (γ-FeOOH) and compare them to an earlier study of goethite (α-FeOOH) and magnetite (Fe₃O₄) precursors. Both the mid-infrared and visible/near infrared spectra of hematite are dependent upon the hematite precursor mineral and the temperature of transformation. Laboratory spectra are compared to spectra from the Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES) and the Mars Exploration Rover (MER) Opportunity Mini-TES and Pancam experiments, allowing us to infer the formation environment of Martian crystalline hematite deposits. Akaganéite and lepidocrocite readily transform to hematite at temperatures of 300 and 500°C, respectively. The visible/near-infrared and mid-infrared spectra of akaganéite-derived hematite are poor matches to data returned from TES, Mini-TES, and Pancam. The spectra of lepidocrocite-derived hematite are slightly better fits, but previously published spectra of goethite-derived hematite still represent the best match to MGS and MER spectral data. The experiments demonstrate that hematite precursor mineralogy, temperature of formation, and crystal shape exert a strong control on the hematite spectra.     

Neotethyan Evolution of the Ankara Mélange,Turkey: An Intraoceanic Subduction-Accretion System

<正>We report here new geochemical and geochronological data from mafic-ultramafic rock suites in the Ankara Mélange in north-central Turkey,and present a new tectonic model for its origin.Considered as one of most important relics of the Neotethyan realm in the region,the Ankara Mélange occurs between the Sakarya Continent     

Isostasie flexurale et érosion différentielle : modélisation numérique appliquée au seuil du Poitou

One uses the principle of flexural isostasy to represent the uplift of the ‘Seuil du Poitou’ in response to erosion. The model indicates that a different uplift of on the last five millions years resulted from an erosion paradoxically stronger in the crystalline basement than in the limestones bedrock. To cite this article: J.-C. Maurin, K. Renaud, C. R. Geoscience 334 (2002) 1149–1155.     

Comprehension and hydrogeological conceptualization of aquifer in arid and semi-arid regions using integrated hydrogeological information system: case of the deep aquifer of Zéramdine-Béni Hassen (east-central Tunisia)

In arid and semi-arid regions, the groundwater overexploitation caused drawdown in piezometric levels and a degradation of chemical water quality. That is why the groundwater monitoring needs a good comprehension of the hydrogeological aquifer properties. This is specially the case of Zéramdine–Béni Hassen deep aquifer (east-central Tunisia). Seismic profiles interpretation highlights the existence of the Zéramdine fault corridor, the Boumerdès anticline, the Moknine and Mahdia grabens that represent lateral boundaries for the study aquifer. The outcrop of the aquifer is located in the Zéramdine, Béni Hassen and Ain Ben Jannet regions, where two lithostratigraphic sections were realized. The piezometric study shows that the principal groundwater flow is from west to east. A secondary flow is from NW to SE. The hydrochemical study of 22 sample shows that the aquifer is characterized by freshwater, dominated by Na–Ca–Cl–SO4 facies. The salinity increase is from the west to the east, which coincides with the principal water flow direction. The integration of all results deduced from the hydrogeophysic, hydrodynamic and hydrochemical studies is developed to investigate hydrological processes of Zéramdine–Béni Hassen aquifer and consequently to propose a conceptual model, which will help to propose a rescue plan for the studied aquifer and to implement a spatial hydrogeological database using the global information system and then to characterize the complex aquifer system.     

Les Pyrénées centrales et orientales au début du Paléozoique (Cambrien s.l.): évolution paléogéographique et géodynamique

In order to debate of the early Paleozoic paleogeography, the repartition of the Hercynian blocks, today scattered around West-Mediterranean Sea. should be known. This is the case for the end of the Paleozoic (Fig. 1), but not for the beginning; Fig. 6 is drawn with the supposed repartition in the middle of the Carboniferous.In Central and Eastern Pyrenees and surrounding areas (Fig. 1), Upper Ordovician beds rest unconformably upon a thick (4–6 km), dominantly pelitic series known as Lower Paleozoic in the Eastern Pyrenees or Seo Formation in the Central Pyrenees. The metamorphic lower part of this series often lies over metagranilic orthogneisses, which are best interpreted as a Precambrian basement, Panafriean-Cadomian in age. By correlation with fossiliferous series of other areas, the Pyrenean Lower Paleozoic should be mainly Cambrian in age (ranging from Uppermost Proterozoic to Lowermost Ordovician).For the purpose of this paper, the complex lithostratigraphic succession of the Lower Paleozoic of the Eastern Pyrenees, with two groups and seven formations, could be summarized (Fig. 2) by a threefold division, from bottom to top: (i) a pelile-greywacke and carbonate unit, with a conspicuous plagioclasic component and a sodic composition (Uppermost Precambrian to Lowermost Cambrian?): (ii) a sandstone-pelite unit, with lithic sandstones, ending with a carbonate level, well developped in the Central Pyrenees (Lower Cambrian?): (iii) a mudstone-siltstone unit (Middle-Upper Cambrian?). Fossiliferous Lower Cambrian beds which outcrop at Terrades (south of the Eastern Pyrenees) could be a remnant of an allochthon unit which can be compared with the nappe-thrusts of the nearby Southern Montagne Noire.The pelite-greywacke and carbonate unit (Fig. 3) occurs only in the South-Eastern Pyrenees as a south to north transgressive platform bordering a basin extending southwards; not far south of Eastern Pyrenees, a volcanism of “intermediate” type supplied in plagioclasic clasts the greywackes and volcanoclastic deposits. Near the base of the sequence, a bimodal volcanism and synsedimentary faults reflect the extensional context of the basin initiation, the geochemistry of which has been related to back-arc setting. An acidic volcanism developped higher in the sequence (tufs and hypovolcanic bodies). Carbonate levels are numerous, particularly in the lower part of the unit. The upper part of the sequence is an oslistostrome made of polygenic intraformational conglomerates fed from the south: it outlines the transition to the next unit.The sandstone-pelite unit (Fig. 4) rests conformably on the previous one in the Eastern Pyrenees, and is unconformable upon the Precambrian basement to the north (North-Pyrenean massifs) and to the west (Central Pyrenees). It is characterized by arkosic lithic sandstones with clear quartz grains: they originated in the erosion of a granitic basement and/or acidic volcanic rocks. Coarseness of the sandstones and thickness (up to 2–4 km) of the unit increase from south-east to north and west. A carbonate upper level, well developped in the Central Pyrenees, can be correlated with Lower Cambrian limestones from the surrounding areas.The mudstone-siltstone unit (Fig. 5) is defined by the prevalence of mm- to cm- scale alternations of argillaceous mud and silt of a flyschoid type, representing a more basinal sedimentation. A carbonate level, the highest in the series, is intercalated in Ihe lower part ot the unit: above this level, deposits are very homogeneous and thiek (about 2 km). A poorly known formation with pelitcs and sandstones caps the muddy-silty unit: it could be Lower Ordovician in age.Thus, the Pyrenean domain shows the same depositional history as West-Mediterranean area: (i) first, a volcano-sedimentary platform or basin occurs, as in Central Spain. Eastern Pyrenees. Sardinia and axial zone of the Montagne Noire, but not further north; (ii) second, a silicoclastic platform spreads out. which becomes carbonated at the end: (iii) third. Ihe basin deepens and receives fine silicoclaslies. This evolution is not fully accounted for in recent synthesis of Pre-hercynian France or Spain, and it should appear useful for a better understanding of the south French Massif Central geological history.     

Serpentinization of mantle formations in the Mauritanides Belt: regions of Agane and Gouérarate (middle-western Mauritania)

Regions of Agane and Gouérarate represent an ancient fragment of ophiolitic suture localized in the axial area of the Mauritanides Belt. These two regions are characterized by the abundance of completely serpentinized formations. In this study, we present the first use of Raman spectroscopy for identifying the species of serpentine present in the Mauritanides Belt. Serpentinites of Agane are derived from refractory peridotites composed of dunites–harzburgites; however, there are also rare serpentinites derived from ultramafic cumulates. Antigorite represents the dominant species in the serpentinite. Furthermore, chrysotile is found as post-antigorite veins. These veins are post-obduction and mark the final phase of serpentinization. The abundance of antigorite and the absence of lizardite confirm that subduction was the environment of serpentinization in these two regions, and that “the oceanic opening” responsible for the formation of ophiolitic sutures in the Mauritanides Belt was limited. The term “serpentinite” is no longer applicable to the formations of Gouérarate. As a result, these formations correspond to old serpentinites transformed to birbirites which are in phase of transformation into laterites.     

Bimodal stable isotope signatures of Zildat Ophiolitic Mélange, Indus Suture Zone, Himalaya: implications for emplacement of an ophiolitic mélange in a convergent setup

Zildat Ophiolitic Mélange (ZOM) of the Indus Suture Zone, Himalaya, represents tectonic blocks of the fragmented oceanic metasediments and ophiolite remnants. The ZOM is sandwiched between the Zildat fault adjacent to a gneissic dome known as Tso Morari Crystalline (TMC) and thin sliver of an ophiolite called as the Nidar Ophiolitic Complex. The ZOM contain chaotic low-density lithologies of metamorphosed oceanic sediments and hydrated mantle rocks, in which carbonates are present as mega-clasts ranging from 100 meters to few centimeters in size. In this work, calcite microstructures, fluid inclusion petrography and stable isotope analyses of carbonates were carried out to envisage the emplacement history of the ZOM. Calcite microstructure varies with decreasing temperature and increasing intensity of deformation. Intense shearing is seen at the marginal part of the mélange near Zildat fault. These observations are consistent with the mélange as a tectonically dismembered block, formed at a plate boundary in convergent setup. The δ¹⁸O and δ¹³C isotope values of carbonates show bimodal nature from deeper (interior) to the shallower (marginal, near the Zildat fault) part of the mélange. Carbonate blocks from deeper part of the mélange reflect marine isotopic signature with limited fluid–rock interaction, which later on provide a mixing zone of oceanic metasediments and/or hydrated ultramafic rocks. Carbonates at shallower depths of the mélange show dominance of syn-deformation hydrous fluids, and this has later been modified by metamorphism of the adjacent TMC gneisses. Above observations reveal that the mélange was emplaced over the subducting Indian plate and later on synchronously deformed with the TMC gneissic dome.     

Résumé

Ohne Zusammenfassung     

The anatomy of an active slide: the Gascons rockslide,Québec,Canada

A railroad crosses the coastal Gascons rockslide, in Quebec, Canada. This study improves the understanding of the rockslide’s failure mechanism and post-failure behaviour responsible for the deformation in the railroad. The slide is an asymmetrical wedge failure of 410,000 m³, in rocks made up of centimetre-scale beds of nodular calcilutite alternating with sandstones and limestone. The post-failure stage of the rockslide is characterised by continuous movement of blocks with speeds ranging from 6 to 110 mm/year. The main water table is just below the sliding surface, but precipitation and snowmelt can raise it above the sliding surface in the upper part of the slide. We propose a model for the rockslide, its failure mechanism, geometry and hydrogeology to provide a baseline for interpreting the near-real-time data collected since 2009.     

Un outil cartographique pour une représentation géosystémique et évolutive des milieux naturels : modélisation d'un réchauffement global

Résumé

Une méthode de cartographie des géosystèmes naturels est exposée. Elle intègre des données fixes dans le temps et l'espace aussi bien que des variables évolutives. Elles permet alors, en accordant ces variables avec des forçages climatiques calculés dans des modèles de réchauffement global, de simuler la répartition nouvelle des géosystèmes, selon l'hypothèse climatique retenue. Un exemple est présenté en Amérique du Nord.